This invention relates to mercury vapor fluorescent lamps and particularly to a method for maintaining the mercury pressure within the lamp at an optimum value by monitoring the current in an optical feedback power supply and regulating the operation of a lamp cooling device in response to current changes.
In a mercury fluorescent lamp, an electrical discharge is generated in mercury vapor at low pressure and typically mixed with argon gas. The light output from the lamp depends, among other variables, on the mercury vapor pressure inside the lamp tube. The primary radiation from the mercury is at 2537 Angstroms and arises from the transition between the lowest nonmetastable excited state and the ground state. This ultraviolet radiation at 2537 Angstroms excites a phosphor which is coated inside the tube walls. The excited phosphor thereupon emits radiation at some wavelength, in the visible spectrum, characteristic of the phosphor.
It is known in the prior art that the optimum mercury pressure for maximum light output of a fluorescent lamp in alternating current operation is approximately 7 mtorr (independent of current) which corresponds to a mercury cold spot temperature of approximately 42.degree. C. At this temperature and pressure, the light output increases monotonically with the current. At cold spot temperatures higher or lower than the optimum, light output falls off.
It is therefore desirable to maintain the mercury pressure at the optimum for any lamp current and at any ambient temperature. Prior art techniques for accomplishing this function required a temperature-sensitive device such as a thermocouple, thermistor or thermostat to monitor the temperature of the cold spot. A feedback circuit provided closed loop control of a temperature-regulating device to maintain the optimum mercury pressure. These methods, although providing a closed loop control of the cold spot temperature, require calibration for each lamp and must rely on a consistent relationship of cold spot temperature to light ouput which may not exist under all conditions.
In copending application Ser. No. 478,746, filed on Mar. 25, 1983 and assigned to the same assignee as the present invention a control method is disclosed which includes an optical detector which senses the visible emission from a fluorescent lamp and generates a signal to change the state of the temperature regulating device. This control method is used in combination with a constant current power supply; e.g. the lamp current is kept constant throughout operation. This technique cannot be used with an optical feedback power supply in which current can vary from very high initial turn-on currents to a minimum current which occurs at optimum lamp operating temperature. In copending application Ser. No. 478,745, filed on Mar. 25, 1983, also assigned to the same assignee as the present invention, another control method is disclosed in which a monitoring circuit detects changes in the lamp arc voltage and generates signals to change the state of the temperature-regulating device. This method can be used with a constant current or an optical feedback power supply. In the case of the optical feedback power supply usage however, there exists a tendency for the voltage sensing circuit to get out of phase on a cyclical basis resulting in less than optimum performance.
The present invention is directed to a novel mechanism and method for maintaining optimum mercury lamp pressure in conjunction with use of a feedback power supply. The mechanism includes a monitoring device for sensing lamp current levels. As will be described in the succeeding descriptive portion of the specification the lamp current is in phase with the light emission of the lamp as a function of temperature. According to one aspect of the invention when a minimum lamp current is established (corresponding to optimum cold spot temperature), any changes in lamp current are detected and used to change the state of the cold spot temperature-regulating device. The lamp current is the continually monitored by a circuit which is adapted to feed back a signal to a cold spot temperature-regulating device under certain conditions. The circuit responds to an increase in the lamp current by reversing the operating mode of the temperature-regulating device. Thus, if the device has been off it is turned on and if on, it is turned off. Either action has the effect of restoring the lamp current to its minimum (optimum) level, and hence restoring the optimum mercury pressure.
A prime advantage of the method of the invention is that once the distribution and feedback circuit are designed with the appropriate algorithm, the system does not require any absolute calibration; that is, the minimum lamp current for a particular lamp does not need to be determined. Further, the feedback circuit is extremely fast relative to the prior art feedback loop which required a longer response time due to the thermal mass of the mercury pool heat sink, the glass envelope and the temperature sensitive device.
The present invention is therefore directed to a control circuit for optimizing and controlling the light output of a fluorescent lamp containing an excess of mercury at a cold spot therein, said circuit comprising:
a feedback power supply for applying operating current to said lamp, said operating current corresponding to a minimum current level associated with an optimum cold spot temperature;
temperature control means adapted to operate in a first mode whereby temperature at said cold spot is increasing and in a second mode whereby temperature at said cold spot is decreasing, and
a monitoring means for detecting an increase in the optimum lamp current, said monitoring means adapted to transmit a signal to said temperature control means changing the instant mode of operation upon detection of said current increase.